Method of manufacturing mnbi magnets



May 2, 1967 A. A. ALDENKAMP 3,317,358

METHOD OF MANUFACTURING MnBi MAGNETS Filed Dec. 2, 1964 FiG.1

FIG.2

INVENTOR. ALBERTUS A.ALDENKAMP United States Patent ware Filed Dec. 2, 1964, Ser. No. 415,535 Claims priority, application Netherlands, Dec. 4, 1963, 301 379 6 Claims. (31. 148-105) This invention relates to methods of manufacturing MnBi magnets in which a mixture of Mn and Bi is melted and heated to a high temperature, whereafter the melt is quenched and the resulting body annealed at a temperature lower than 450 C. and pulverized after cooling. Subsequently the MnBi-particles thus obtained are agglomerated by heating and then compressing them.

In a known method of this kind, the mixture is melted and heated to a temperature higher than l,-000 C., for example, to 1,250 C. Nextthe melt is quenched, for example, in cooled metal molds. This method results in a yield of the magnetic MnBi compound of approximately 90%, it being possible in certain cases to obtain a yield higher than 90%, but not higher than approximately 92% to 93%.

The method according to the invention consists in that the quenching process is effected by granulating the melt from a temperature between approximately 1,000 C. and 1,250 C. so as to obtain a yield of the magnetic MnBi compound higher than 94% in a reproducible manner. In most cases a yield is obtained which is higher than 95% with peak values from 98% to nearly 100%.

The melt may be granulated with the use of a granulating machine which utilizes rotary discs. Such machines and also other machines suitable for this purpose are described inter alia in the book Fundamental Principles of Powder Metallurgy by W. D. Jones, published for the first time in 1960.

By the granulating process it is possible to obtain substantially spherical grains of a diameter less than approximately 1.5 mms. The Mn and the Bi are divided in these grains with much greater fineness than in the quenching methods hitherto used, for example, the said quenching method in cooled metal molds. It is this fine division which gives rise to the said high yield of the magnetic MnBi compound.

When using the method according to the invention it has been found that the yield of the magnetic compound falls off if granulating is effected at a temperature higher than approximately 1,000 C. For example, a yield of the magnetic compound from 98% to nearly 100% may be obtained when granulating from 1,000 C. and a yield of 95% when granulating from 1,250 C. Also it has been found that the said results can be obtained only if the annealing temperature of the grains is lower than 360 C.

If the granulating process takes place at a temperature lower than 1000 C., for example between 800 C. and 900 C., the melt still contains a solid phase. The said temperatures cannot therefore be used for the method according to the invention.

As to the annealing temperature lower than approximately 360 C. it is mentioned that for obtaining the said higher yield of the magnetic MnBi compound longer annealing times are of course necessary at lower temperatures. If annealing takes place, for example, at a temperature of approximately 240 C. the annealing period is for example 48 hours. If annealing is effected at approximately 330 C. the desired yield is obtained after, for example, 6 hours.

It has also been found that, upon annealing at a temperature higher than 360 C., crystallographic changes presumably occur in the MnBi compound and also dissociation of this compound takes place, which gives rise to a lower magnetic moment in the MnBi compound so that, after agglomerating the particles obtained by the pulverisation of the grains, a permanent magnet results having magnetic properties which are not interesting in practice.

It is also important that the granulating process is effected in an atmosphere protecting the MnBi compound from oxidation, for example, in an inert gas such as nitrogen or argon, while a non-oxidizing liquid, for example, a hardening oil is used as a quenching agent.

The invention, it will now be described with reference to the accompanying drawing, in which:

FIGURE 1 is a cross-sectional view of a granulating machine for carrying out the method according to the invention, and

FIG. 2 is a plan view on a component part of this machine, as viewed in a direction indicated by the arrow II-II.

The machine of FIGURE 1 comprises a double-walled vessel having an outer wall 1. The space between the outer wall 1 and an inner wall 2 is filled through a pipe 3 with a hardening oil which flows over the inner wall 2 into the vessel 4. This vessel contains a rotor 5 com- .prising a double blade 5a and a disc 5b (see also FIGURE 2). The rotor is rotated by an electric motor 6 at 2800 rev./min. The hardening oil is pumped up from an oil container 7 by a pump 8. To obtain better thermal conductivity, the oil is used at a temperature of 65 C. The oil flows through a pipe 9 not only to pipe 3 but also to a spraying head 10. Approximately 60 litres of oil per minute are sprayed onto the rotor 5.

A crucible 11 contains a homogeneous manganese-bismuth melt 12 (50/ 50 at. percent). The molten material is likewise poured through a pipe 13 onto the rotor 5. The jet of the molten MnBi material as well as that of the hardening oil is periodically interrupted by the blades 5a of the rotor 5. After passing the blades 5a, the two materials are flung by centrifugal force via the disc 5b to the inner wall 2. The MnBi material thus coagulates to grains less than 3 mms. in diameter. The material thus granulated falls down in the vessel 4 and, after the machine has been stopped and, for example, its cover 14 taken off, may be removed upon reaching a determined level. The oil present above the granulate flows through an overflow channel 15 back to the container 7.

To prevent oxidation of the melt and combustion of the hardening oil, approximately 10 liters of nitrogen per minute are blown through a pipe 16 into the machine, leaving it again through the pipe 13.

After purifying the granulate, it is annealed at 320 C. for 15.5 hours, so that a yield of the magnetic MnBi compound between 94% and nearly is obtained.

What is claimed is:

1. In the method of manufacturing an MnBi magnet the steps of forming a melt of a mixture of Mn and Bi, forming granules from the melt at a temperature between approximately 1000 C. and 1250 C., quenching said granules, and annealing said granules at a temperature lower than approximately 360 C.

2. In the method of manufacturing an MnBi magnet the steps of forming a melt of a mixture of Mn and Bi, forming granules from the melt at a temperature between approximately 1000 C. and 1250 C. in a non-oxidizing atmosphere, quenching said granules, and annealing said granules at a temperature lower than approximately 360 C.

3. In the method of manufacturing an MnBi magnet the steps of forming a melt of a mixture of Mn and Bi,

forming granules from the melt at a temperature between approximately 1000 C. and 1250 C. in an inert gas atmosphere, quenching said granules, and annealing said granules at a temperature lower than approximately 360 C.

4. In the method of manufacturing an MnBi magnet the steps of forming a melt of a mixture of Mn and Bi, forming granules from the melt at a temperature between approximately 1000" C. and 1250 C. in an argon atmosphere, quenching said granules, and annealing said granules at a temperature lower than approximately 360 C.

5. In the method of manufacturing an MnBi magnet the steps of forming a melt of a mixture of Mn and Bi, forming granules from the melt at a temperature between .4 approximately 1000 C. and 1250 C. in an inert gas atmosphere, quenching said granules in a hardening oil, and annealing said granules at a temperature lower than approximately 360 C.

6. In the method of manufacturing an MnBi magnet the steps of forming a melt of a mixture of Mn and Bi, forming granules from the melt at a temperature between approximately 1000 C. and 1250 C. in an inert gas atmosphere, quenching said granules in a hardening oil, and annealing said granules at a temperature of about 320 C.

No references cited.

DAVID L. RECK, Primary Examiner.

W. W. STALLARD, Assistant Examiner. 

1. IN THE METHOD OF MANUFACTURING AN MNBI MAGNET THE STEPS OF FORMING A MELT OF A MIXTURE OF MN AND BI, FORMING GRANULES FROM THE MELT AT A TEMPERATURE BETWEEN APPROXIMATELY 1000*C. AND 1250*C., QUENCHING SAID GRANULES, AND ANNEALING SAID GRANULES AT A TEMPERATURE LOWER THAN APPROXIMATELY 360*C. 